Automatic transmissions typically include input clutch units that have a piston that engages and releases a pack of friction disks to engage and disengage a gear set. Referring to FIG. 1, a portion of a cast aluminum transmission case is indicated generally at 10. Located within case 10 is a cylindrical steel piston 12. High pressure oil must be supplied from outside of case 10 and through piston 12 in order to drive the piston 12. Case 10 has a boss 14 through which an inlet 16, which is generally in the form of a stepped cylinder, is machined. Inlet 16 opens to a portion of the outer surface of piston 12 which, since the diameter of inlet 16 is significantly smaller that the diameter of piston 12, in effect forms a generally planar bottom wall to inlet 16. A central port 18 drilled through piston 12 is generally coaxial to inlet 16. The outer portion of piston 12 does not move appreciably relative to inlet 16. The supply line for pressurized oil includes a pipe 20 that fits centrally within inlet 16, with a formed circular ridge 22 that acts as a stop member. At a location on case 10 not illustrated in FIG. 1, pipe 20 makes a ninety degree bend, and is held down by a strap type retainer bolted to case 10 that prevents pipe 20 from pulling out of inlet 16.
Not only is it necessary to physically retain pipe 20, sealing must be provided around pipe 20 both to prevent the high pressure oil that it supplies from escaping between the bottom of inlet 16 and piston 12, and to also prevent it from escaping through the top of inlet 16. The known seal shown in FIG. 1 provides those two necessary sealing functions with two separate, independent structures. The first sealing function is provided by a lower seal assembly designated generally at 24. The lower seal assembly 24 includes a metal sleeve 26 to which a rubber compression seal 28 is molded. Before pipe 20 is added, sleeve 26 is press fitted into the lower portion of inlet 16 until seal 28 is sufficiently compressed against piston 12 in surrounding relation to port 18. The seal 28 would not, as a practical matter, be visible to the installer during this process. Therefore, unless a pressure sensitive tool were used to insert sleeve 26 and stop it when some threshold or reference pressure was reached, then the necessary compression of seal 28 would have to be achieved and limited by some other means. Possible methods could include stopping the insertion of sleeve 26 when its upper edge reached a defined reference point on the inner surface of inlet 16, or monitoring the distance between the top edge of sleeve 26 and the piston 12, and stopping the insertion of sleeve 26 at the value of that distance where the compression of sleeve 28 should have reached the correct value. Any such means of limiting and defining the compression of seal 30 would inevitably be at least somewhat dependent on how well the manufacturing tolerances were held between the piston 12 and the reference point on the inlet 16, or between the seal 28 and the top edge of sleeve 26, or both. While those tolerances can be held sufficiently closely, it would clearly be easier and potentially less costly to have a seal that could tolerate less precisely held tolerances. Furthermore, the amount of the compression of seal 28, which determines its sealing effectiveness, would be essentially invariant once seal 24 was installed. The other necessary sealing function is provided by an O-ring 30, which is added after seal 24. O-ring 30 is compressed between a lower washer 32 that is first seated on the step of inlet 16 and an upper washer 34 that is seated on top of O-ring 30 and which is pressed downwardly by the pipe ridge 22 after pipe 20 is fastened in place. Thus, several steps are necessary to install all the sealing structures.